nested list comprehensions

Question

I tried to use the value of an outer list comprehension in an inner one:

[ x for x in range(y) for y in range(3) ]

But unfortunately this raises a NameError because the name y is unknown (although the outer list comprehension specifies it).

Is this a limitation of Python (2.7.3 and 3.2.3 tried) or is there a good reason why this cannot work?

Are there plans to get rid of the limitation?

Are there workarounds (some different syntax maybe I didn't figure out) to achieve what I want?

Answer 1

You are talking about list comprehensions, not generator expressions.

You need to swap your for loops:

[ x for y in range(3) for x in range(y) ]

You need to read these as if they were nested in a regular loop:

for y in range(3):
    for x in range(y):
        x

List comprehensions with multiple loops follow the same ordering. See the list comprehension documentation:

When a list comprehension is supplied, it consists of a single expression followed by at least one for clause and zero or more for or if clauses. In this case, the elements of the new list are those that would be produced by considering each of the for or if clauses a block, nesting from left to right, and evaluating the expression to produce a list element each time the innermost block is reached.

The same thing goes for generator expressions, of course, but these use () parenthesis instead of square brackets and are not immediately materialized:

>>> (x for y in range(3) for x in range(y))
<generator object <genexpr> at 0x100b50410>
>>> [x for y in range(3) for x in range(y)]
[0, 0, 1]